They are replacing the stop in Mazatlan with a second day in Cabo San Lucas. I have been on this cruise, trust me... Cabo is a much better place to go. Some people are also choosing to stay overnight in Cabo (at their expense) instead of returning to the ship. But having 2 days to explore Cabo is not a bad trade off.
Having said that, the situation in Mazatlan could change by the time you take your cruise in April and they may stop there for your cruise. Carnival is working closely with the authorities and will resume the port when things are safe. Note that the cruise passengers were never in danger but its better to be safe than sorry is the stance Carnival has taken.
As for your other questions, the lines were not bad. Getting off the ship at the end of the cruise was a bit confusing because the baggage claim and customs clearance procedures were not the best organized. But I have heard that when John Heald was the cruise director, he helped change tings up in order to move things along, so maybe that problem is resolved.
Other than that it was a Fantastic cruise. I would go again even without Mazatlan.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013